It is known to coat a synthetic-resin foam body with electrically conductive particles for use of the body in a filter, as a seal, or as a radiation- or sound-damping element.
The simplest method of making such a body entails mixing the particulate and conductive filler material with the resin before foaming same. The resultant material has a somewhat greater conductivity and considerably greater density than the foam material itself, but still has a conductivity which is only a very small fraction of that of the conductive particulate material mixed with the resin. The main reason for this is that the synthetic resin completely encapsulates the conductive particles so that the flow of electricity or heat between particles must be effected across these virtually nonconductive bridges. At the same time such a standard conductively filled synthetic-resin body is considerably less elastic than the body would be absent such filling, and is in fact greatly more fragile and liable to tearing or permanent deformation when crushed. As the amount of conductive particles is increased in order to maximize the conductivity of the body, the physical properties of the body becomes even worse. A similar process is described in German patent publication No. 1,719,053.
It is also known to form such a body, particularly usable as a seal, with relatively large conductive particles or balls embedded in it. In use the body is compressed greatly so that the relatively incompressible conductive balls are pressed together and effectively crush the resin webs separating them leaving the balls in good electrical contact with one another. The resultant body is relatively conductive, but is ofter unsatisfactory because the damage done to the synthetic-resin structure to achieve this good conductivity has a highly deleterious effect on the physical properties of the body.
It is also known from German patent No. 1,569,416 filed with a claim to the priority of U.S. patent application 365,793 of 7 May 1964 to shake up a synthetic-resin foam body and small conductive particles in a bag, then to fill this bag with a mixture of propane and oxygen. The gas mixture in the bag is then exploded to embed the particles in the body. Such a procedure increases the overall weight of the body by approximately 76%, using a polyurethane foam and graphite particles. The overall specific resistance of such a body is decreased with such a treatment by a factor of between 5.times. 10.sup.3 and 5.times. 10.sup.4. Nonetheless the resultant body still has a specific resistance approximately 2.times. 10.sup.10 greater than the resistance of the graphite itself. Thus it can be assumed that in the finished product according to this patent the particles are nonetheless separated sufficiently to account for this considerable resistance.
German patent publication No. 1,704,781 published 27 May 1971 describes another arrangement for coating a synthetic-resin foam body with tiny particles of a thermoplastic synthetic resin, so that the resultant body can easily be heat-sealed or welded to a support. Such a method does not, however, suggest a method of making an improved conductive foam, as the behavior of particles of polyethylene, for instance, and copper can hardly be compared.